Acceleration sensors are used in fields in which it is essential to measure motion, such as, for example, of car crash safety testing, robots, transportation equipment, equipment relating to nuclear power generation, ships, space and aeronautical equipment, micro-motion devices and the like. In order to maintain the accurate measurement by such acceleration sensors, it is necessary to periodically calibrate the acceleration sensors. Conventionally the most reliable method for this calibration has been to affix the acceleration sensor to a mounting table, use a laser interferometer to measure the motion of the mounting table, and compare the measured value with the output value of the acceleration sensor affixed to the mounting table.
However, in the above conventional acceleration calibration technology, no technology has yet been established concerning the measurement of dynamic linearity at an acceleration level and within a frequency range required by industry, so the situation is that there is as yet no international standard provided for the technology.
There exists a piezoelectric acceleration sensor used as a conventional acceleration sensor. A feature of the piezoelectric acceleration sensor is that it can measure up to very high shock acceleration; this product is on the market and is extensively used. However, with respect also to this piezoelectric acceleration sensor, no technique has been proposed for measuring dynamic linearity, which is a very important aspect of dynamic measurement. Consequently, it is not at all known if dynamic linearity has actually been established up to a high acceleration level in the order of 106 m/s2, so on this point, reliability of the sensor is inadequate.
In the field of acceleration detection, there are seismographs. In the measuring of acceleration at low frequencies, such as in earthquake measurement, although it is necessary to evaluate the effect of parasitic rolling vibration of the mounting table, such evaluation is difficult, and since at the same time it is also difficult to realize low frequency vibration with high linearity, acceleration sensors able to measure acceleration from a direct current component are still only calibrated statically.
Therefore, with respect to acceleration sensors utilized in important fields of industry, an object of the present invention is to provide a method and apparatus for accurately and easily measuring the dynamic linearity of this acceleration sensor over a wide range, from acceleration values generally utilized in such fields up to acceleration in the order of 106 m/s2, and as a result, making it possible to accurately and easily calibrate the acceleration sensor.
The present invention resolved the above problems in accordance with the basic concept described below. That is, in the general field of measurement technology, accurate measurement cannot be achieved unless linearity is established. Linearity is also important in dynamic measurement, but with respect to gain and phase, generally it is not easy to verify dynamic linearity. However, as a general definition, when there is an output signal X(t) for an input signal x(t) and an output signal Y(t) for an input signal y(t), using arbitrary constants a and b, dynamic linearity is established if the output signal for an input signal a·x(t)+b·y(t) becomes a·X(t)+b·Y(t).
The acceleration sensor that is the measurement object is attached to the end surface of the metal rod. Acceleration of the end surface produced when an elastic wave pulse generated by an inner projectile launched from the inner launch tube to impact the rod end surface reflects at the other end surface, that is, input acceleration signal ain,1(t) to the acceleration sensor and acceleration sensor output aout·1(t) are obtained. Next, acceleration of the end surface produced when an elastic wave pulse generated by an outer projectile launched from the outer launch tube to impact the rod end surface reflects at the other end surface, that is, input acceleration signal ain,2(t) to the acceleration sensor and acceleration sensor output aout·2(t) are obtained. Instead, an outer projectile may be launched from an outer launch tube first, and then an inner projectile from an inner launch tube.
Finally, when an inner projectile and an outer projectile are made to simultaneously impact the metal rod, from the linearity of the elastic wave, the acceleration signal acting as an input signal to the acceleration sensor will be ain,1(t)+ain,2(t). The acceleration sensor output signal at this time is taken to be aout(t). If linearity is established, since the output signal will be aout·1(t)+aout·2(t), by comparing this signal with aout(t) in frequency domain or time domain, a method of measuring the dynamic linearity of the acceleration sensor with respect to gain and phase is attained, and an apparatus implementing the method. The projectile from the inner launch tube and the projectile from the outer launch tube when the above projectiles are launched simultaneously have the same shape as in the case where each is launched on its own, and conditions, such as launch pressure, initial position inside the launch tube and so forth, have to be the same.